be somebody's pigeon

be (one's) pigeon

To be one's area of expertise or responsibility. I can't approve expense reports, but Betty can—that's her pigeon.
See also: pigeon

be somebody’s pigeon

(old-fashioned, British English) be somebody’s responsibility or business: Somebody needs to write a report on training for the manager, but it’s not my pigeon.Gustav will have to tell them first, it’s his pigeon.
See also: pigeon